Field of the Disclosure
The disclosure generally relates to methods and devices for concentrating wastewater and more particularly to methods and devices for concentrating dissolved salts in flowback and produced water from natural gas wells.
Background
Natural gas is a naturally occurring hydrocarbon gas having many beneficial uses. Natural gas is an emerging world-wide energy source that may be used for electrical generation and vehicle propulsion, among other uses. Natural gas is also useful as a chemical feedstock in the manufacture of plastics and other organic chemicals. Natural gas is most often found deep underground and thus, must be extracted by drilling a well. After the well is drilled, the natural gas seeps into the well bore, where it can be removed and stored for refining and future use.
As more easily accessible natural gas formations are mined out, natural gas producers are turning more and more to natural gas rich rock formations, such as shales, which have a matrix permeability that is too low to allow gas to flow through, or out of, the formation. Hydraulic fracking was developed to access natural gas contained in these formations. Hydraulic fracking involves pumping large volumes of water, often under pressure, into these formations in an attempt to create small cracks in the rock formation, which allow the natural gas to flow back into the well bore, where it can be extracted. Once the rock formation is fracked, the natural underground pressure may be sufficient to force the natural gas into the well bore. Alternatively, to enhance natural gas collection, additional water may be forced into the well under pressure to increase the down bore pressure, which causes natural gas to displace into the well bore more quickly.
One way to enhance down bore pressures is to increase the density of the liquid that is pumped down the well. Most often, density of the fracking water is increased by mixing salt into the water until the water is close to, or at, the saturation point for the salt. The particular salt also influences the final density of the fracking water. Artificially increasing the density of the fracking water in this manner, while accomplishing the main goal of increasing downbore pressure, is relatively expensive, which increases the costs of extracting the natural gas, often far beyond the point of economic feasibility.
The natural, or enhanced, down bore pressures also cause some of the fracking water (known as flowback water) to also seep back into the well bore. Additionally, naturally occurring ground water may also seep into the well bore (after a certain amount of time, this type of water is called produced water). Because the flowback waters and produced waters were in contact with mineral formations deep underground, these two types of wastewater contain many types of dissolved solids (such as salts) and suspended solids, such as silica, which must be processed in accordance with environmental regulations.